The high biomass production level of sweet sorghum jointly with a good sugar content make this crop suitable as an efficient renewable energy resource. However, to fully explore the potential of sweet sorghum for the bioethanol supply chain, it is necessary to maximize the energy performance and efficiency rather than the productivity of the crop. In this field experiment, the sweet sorghum was cultivated under three different energy input levels (HIGH, MID and LOW), modulating the nitrogen doses and varying the soil management. Although the biomass production was slightly reduced under middle and low energy input (24.87 t ha-1 of dry biomass, as mean) if compared with the highest energy treatment (26.15 t ha-1 of dry biomass), the energy performance and efficiency resulted greatly improved (+ 35% for the net energy gain and + 76% for the energy ratio in favour of LOW compared to HIGH treatment). Finally, the simulated greenhouse gas (GHG) emissions to produce 1 MJ of energy from the whole bioethanol supply chain resulted more than halved when comparing the extreme treatments (20.9 vs 8.1 g MJ-1, in HIGH and LOW, respectively). Keywords: Greenhouse gases; energy balance; energy efficiency; energy crops.